Derivation of conventional crystallographic descriptions of new phases from results of ab initio inorganic structure modelling

The results of ab initio inorganic structure modelling are often in the form of Cartesian coordinates of atoms in a large, periodical and in general oblique simulation box containing hundreds to thousands of atoms. The contents of that box may correspond to a single-crystal, a twin, a mixture of phases or a disordered block of matter. The problem of extracting corresponding crystallographic descriptions for single-crystal regions in the box, a necessary step in view of full quantum calculations and publication, is different from the familiar problem of extracting crystal symmetry and structure from experimental diffracted intensity data. The deductive computer-aided method developed at the National Research Council of Canada over the years is based on eye identification of three pairs of atoms related by conjugate translations in the same single-crystal region on a stereo plot, followed by derivation of fractional coordinates for the atomic content of the corresponding primitive cell. Running this data through the MISSYM program discloses potential symmetry elements of the structure, with their corresponding crystallographic directions. These elements are then critically examined and accepted either as symmetry or pseudosymmetry on the basis of comparison of coordinate deviations between related atoms with the expected magnitude of thermal motion. All calculations described here can be performed with the NRCVAX system of programs.